Coatings are often applied to surfaces of glass or ceramic substrates to make the surfaces scratch-resistant or for other reasons. Examples of methods used to apply such coatings are physical vapor deposition and chemical vapor deposition. In one example coating setup, substrates are mounted on an exterior surface of a rotating drum, which is arranged vertically inside a vacuum chamber. Sputtering sources are arranged around the rotating drum. During the coating process, the drum is rotated about a vertical axis. The sputtering sources deposit atoms of coating material on the substrates as the drum rotates. One of the important considerations in this setup is how to mount the substrates on the rotating drum and keep the substrates secured on the rotating drum during the film deposition.
A simple method of mounting substrates on a rotating drum may be to tape or bond the substrates to the drum. With taping, the substrates will be slightly raised off the mounting surface of the drum by an amount equal to the thickness of the tape, making it possible for particles to accumulate at the backside of the substrates where the tapes are applied, which may result in undesired or non-uniform coating at the backside. In the case of bonding, there will be extra process steps needed to release the substrates from the drum as well as remove any adhesive residues from the substrates, all without damaging the substrates. If it is desired to coat both sides of the substrates, all these extra process steps would need to be carried in between coating both sides.
Another method that may be used to mount substrates on a rotating drum involves use of clamps that grip the edges of the substrates. In this case, the areas of the substrates gripped by the clamps will not receive the coating material, leading to non-uniform coating of the substrates. Also, the clamps can shield areas of the substrates from the coating atoms as the drum rotates, leading to further non-uniform coating of the substrates.
Another method commonly used to hold substrates is vacuum chucking. In vacuum chucking, small holes in the chuck behind the substrate provide a means to apply vacuum between the substrate and chuck, and atmospheric pressure exerted on the substrate provides the holding force. However, thin film deposition processes are often carried out in high vacuum (˜10−5 Torr) to prevent unwanted molecular species or particles from being trapped under the coating. Vacuum chucking is not effective in high vacuum plasma because the pressure differential exerted downward on the substrate is non-existent.
In the semiconductor fabrication industry, electrostatic chucking is commonly used to hold wafers during lithography processes. An electrostatic chuck typically includes an electrode encapsulated in an alumina ceramic disk. When a substrate, semiconductor or dielectric, is brought into contact with the alumina, the charge on the encapsulated electrode induces a charge polarization in the substrate, which then is electrostatically attracted to the electrode and held firmly in place by that charge. De-chucking is accomplished by turning off power to the electrode, which removes the induced polarization. To prevent permanent polarization in the substrate, polarity on the electrodes is frequently switched and power to the electrodes is pulsed.
Electrostatic chucking does not suffer from the challenges discussed earlier for the other chucking methods. However, electrostatic chucking faces a different kind of challenge for the rotating-drum-type coating process, i.e., how to maintain chucking and electrical interconnect to the chucking electrodes as the drum rotates rapidly. Similarly challenges may be faced in horizontal and vertical inline coating systems where the substrates have to be mounted on a carrier that is moving horizontally or vertically.